1. Field of the Invention
The present invention relates to a radio system comprised of radio transmitting apparatuses and radio receiving apparatuses such as a mobile communication system constituted by a plurality of mobile stations and radio base stations, and in particular, to an SIR estimation method of, from a desired signal level and an interference signal level of a received signal, acquiring an SIR value of the received signal, and a transmission power controlling method for controlling a transmission power of the radio transmitting apparatus by using the SIR value measured by the SIR estimation method.
2. Description of the Prior Art
In recent years, CDMA (Code Division Multiple Access) communication mode which is good at handling interference and hindrance is receiving attention as a communication mode to be used for a mobile communication system. In the CDMA communication mode, a transmitting apparatus spreads a user signal by using a spreading code and transmits the user signal. Then, a receiving apparatus despreads the received signal by using the same spreading code to obtain the original user signal. A plurality of transmitting apparatuses spread user signals by using different spreading codes, and each of receiving apparatuses can specify the communication by selecting the spreading code to be used when despreading the received signal. Therefore, the same frequency band can be used for a plurality of communications.
In the CDMA communication mode, if spreading codes to be used in mobile stations are mutually completely orthogonal, signals transmitted by the stations do not interfere with other signals. However, it is difficult to keep all the spreading codes to be used completely orthogonal, and so each spreading code does not become completely orthogonal but has correlation components between it and other codes in reality. Thus, these correlation components become interference components for the communication, which are a deterioration factor of the communication quality. The interference components are generated due to such a factor, so that they increase as the number of communications increases. For that reason, if transmission powers of all mobile stations are a fixed value, there arises a situation wherein a signal from the mobile station near a base station is so strong that it significantly interferes with the signal from a distant mobile station, which is a so-called near-far problem. Therefore, transmission power control is performed in order to set the transmission power of an up link from each of the mobile stations to the base station at an appropriate value.
Transmission power control (hereafter, referred to as TPC) will be described. One station measures an SIR (Signal to Interference power Ratio)based on a received signal from the other station, and then compares the measured SIR to a predetermined SIR (hereafter, referred to as a target SIR ). In the case where the measured SIR is smaller than the target SIR, the other station is instructed to increase the transmission power, and in the case where the measured SIR is larger than the target SIR, it is instructed to decrease the transmission power. To perform measurement of the SIR, a desired signal level (hereafter, referred to as RSSI (received signal strength indicator)) of each path after despread is divided by an interference signal level (hereafter, referred to as ISSI (interference signal strength indicator)), and then the SIRs of all the paths are added up. The ISSI is the sum of the noise inputted to an antenna, other user signals and noise generated in the station.
FIG. 11 shows a block diagram of a radio base station 90 and a mobile station 94 for performing such transmission power control. In addition, a data format transmitted and received between the base station and the mobile station is shown in FIG. 12. Data 1 and data 2 in FIG. 12 are user data respectively, and TPC bits are a transmission power control request from the base station 90 or the mobile station 94.
The radio base station 90 is comprised of a transmitting portion 91, a receiving portion 92 and an SIR measuring portion 93, and the mobile station 94 is comprised of a transmitting portion 95, a receiving portion 96 and an SIR measuring portion 97.
At the radio base station 90, the receiving portion 92 receives the signal from the mobile station 94, and the SIR measuring portion 93 measures the SIR of the received signal and determines whether or not the measured SIR is a preset target SIR or more. And in the case where the measured SIR is smaller than the target SIR, the SIR measuring portion 93 sends to the transmitting portion 91 TPC information for requesting an increase in transmission power, and in the case where the measured SIR is the target SIR or more, the SIR measuring portion 93 sends to the transmitting portion 91 the TPC information for requesting a decrease in transmission power. The transmitting portion 91 transmits to the mobile station 94 data including the TPC information as the TPC bits. And the mobile station 94 having received the signal from the radio base station 90 decreases or increases the transmission power of the transmitting portion 95 based on the TPC bits included in the received signal so that the transmission power control is performed. While it was described by using the case of transmitting the TPC bits from the radio base station 90 to the mobile station 94, the same method is also used in the case of transmitting the TPC bits from the mobile station 94 to the radio base station 90.
FIG. 13 shows a configuration of the conventional radio base station for performing such transmission power control. The radio base station shown in FIG. 13 comprises a radio portion 63, communication channel circuits 1121 to 112K provided to each channel, a common control circuit 111 for controlling these communication channel circuits 1121 to 112K and a reference frame generating circuit 64.
The radio portion 63 demodulates the signal received by an antenna and outputs the demodulated base band signal as a signal R1. The radio portion 63 combines transmitting signals T1 from the communication channel circuits 1121 to 112K, and then modulates the combined signal thereafter to transmit it.
The reference frame generating circuit 64 generates and outputs a reference frame signal 26 which is the signal to be a reference of frame timing.
The common control circuit 111 controls the communication channel circuits 1121 to 112K by using sector selection signals 20, control signals 23 and target SIR notification signals 24.
In case of starting the communication of one of the communication channel circuits 1121 to 112K, the common control circuit 111 specifies a sector (may be a plurality) to be used for that communication channel circuit by means of the sector selection signals 20, and further switches the control signal 23 corresponding to the communication channel circuit starting the communication from 0 to 1, that is, to be active.
As shown in FIG. 14, the communication channel circuit 1121 comprises an A/D converter 72, a D/A converter 73, an SIR estimating portion 141 and a TPC bit insertion circuit 74. The configuration of each of the communication channel circuits 1122 to 112K is the same as that of the communication channel circuit 1121 shown in FIG. 14.
The A/D converter 72 converts the analog received signal R1 from the radio portion 63 into a digital signal. The SIR estimating portion 141 performs a path search based on the digital signal from the converter 72 and outputs a user received signal by despreading the digital signal. The SIR estimating portion 141 estimates the SIR of the user received signal, and then generates TPC information 25 based on the estimated SIR.
The TPC bit insertion circuit 74 inserts the TPC information 25 as TPC bits in an inputted user transmitting signal, and sends it to the D/A converter 73. The D/A converter 73 converts the user transmitting signal including the TPC bits into an analog signal so as to output it as the transmitting signal T1.
FIG. 15 shows a configuration of the SIR estimating portion 141 shown in FIG. 14. As shown in FIG. 15, the SIR estimating portion 141 comprises a path search circuit 2, a selector circuit 3, a despreading circuit 4, an ISSI estimation circuit 145, an RSSI estimation circuit 6, an SIR estimation circuit 7, an SIR addition circuit 8 and an SIR determination circuit 9.
The SIR estimating portion 141 has the control signal 23 supplied thereto from the common control circuit 111 shown in FIG. 13, and the SIR estimating portion 141 performs the following operation in the case where the control signal 23 is “1,” and performs no operation in the case where the control signal 23 is “0.”
The received signals R1 from M antennas are inputted to the path search circuit 2 and selector circuit 3 via the A/D converter 72. The signal R1 (y, t) [y=1, 2, . . . , M] means the signal from an antenna y of the M antennas when the time is t, and one antenna is provided to each sector. The M sector selection signals 20 inputted to the path search circuit 2 indicate which sector the path search is to be performed in, and are represented as SCT (1), SCT (2), . . . , SCT (M). In the case where the sector in which the path search is performed is a sector m, SCT (m) of the sector selection signals 20 is “1” and the sector selection signal corresponding to the sector in which the path search is not performed is “0.” The path search can be performed in a plurality of sectors.
The path search circuit 2 takes in the signal R1 corresponding to the sector selected by the sector selection signals 20 (signal R1 (3, t) in the case where the selected sector is the sector 3, for instance), and detects correlation values between this signal and a predetermined code in reference to the reference frame signal 26 showing frame timing so that it selects N paths in the decreasing order of the correlation value. Results of the selection are outputted in parallel or serially as path information (1) to (N) shown in FIG. 16. The path information (1) to (N) includes source information and delay information. The source information indicates which sector the received signal is inputted from, and the delay information indicates how much the received signal is shifted in time from the timing shown by the frame signal. As each antenna normally has multi-path fading performed thereto, it may happen that a plurality of paths are selected from the same source. A concrete example of the path information outputted from the path search circuit 2 is shown in FIG. 17.
As shown in FIG. 18, the selector circuit 3 comprises M:1 selectors 411 to 41N, and the signals R1 (R1 (1, t) to R1 (M, t)) are inputted to each of the selectors 411 to 41N. Each of the selectors 411 to 41N selects the signal (sector) specified by the source information of the inputted path information, and outputs it as the signal R2 (R2 (1, t) to R2 (N, t)) to the despreading circuit 4.
As shown in FIG. 19, the despreading circuit 4 comprises multipliers 421 to 42N, a code generator 44 and a delay circuit 43. The code generator 44 generates a predetermined code of code length L in synchronization with the reference frame signal 26, and the delay circuit 43 delays the inputted code by the value of the delay information of each of the path information (1) to (N). Each of the multipliers despreads the inputted signal R2 by multiplying the inputted signal R2 by the code from the delay circuit 43. The signals R3 (R3(1,t) to R3(N,t)) outputted from the multipliers are inputted to the ISSI estimation circuit 145 and the RSSI estimation circuit 6. In addition, each of the signals R3 is outputted as the user signal.
The ISSI estimation circuit 145 starts the ISSI estimation using a preset value as an initial value for turning the control signal 23 from “0” to “1.” When performing the ISSI estimation of each of the signals R3, the ISSI estimation circuit 145 refers to the source information included in the path information (1) ˜(N) to identify which sector each of the signals R3 is from, and then performs the ISSI estimation with the preset value as the initial value. The ISSI estimation circuit 145 periodically estimates the ISSI of each of the signals R3, and adds it and an immediately preceding ISSI value as a weight and outputs the result thereof as an ISSI signal to the SIR estimation circuit 7. The RSSI estimation circuit 6 periodically estimates the RSSI of each of the signals R3, and outputs the result thereof as an RSSI signal to the SIR estimation circuit 7.
The SIR estimation circuit 7 acquires sir of each of the N paths (each signal R3) by dividing each of the RSSI signals from the RSSI estimation circuit 6 by a corresponding ISSI signal of the ISSI signals from the ISSI estimation circuit 145 (“RSSI (1)/ISSI(1)”for instance). The SIR addition circuit 8 acquires the SIR by adding up the sir of each of the N paths.
The SIR determination circuit 9 compares the SIR from the SIR addition circuit 8 to the target SIR provided from the common control circuit 111, and outputs the TPC information 25 to the TPC bit insertion circuit 74. The TPC information 25 is “0” if the measured SIR is the target SIR or more, or it is “1” if smaller. Here, “1” is the signal for requesting the other station to increase the transmission power, and “0” for requesting it to be lowered.
Next, the operation of the SIR estimating portion 141 in this conventional radio base station will be described.
First, the path search circuit 2 detects the N paths by detecting the correlation values between the signal R1 corresponding to the sector specified by the sector selection signals 20 from the common control circuit 111 and the predetermined codes, and then outputs the detected results as the path information (1) to (N).
For instance, in the case where the sectors specified by the signals 20 are sectors 4 and 5 (M=5), the sector selection signals 20 are represented by the following expression (1).
                              SCT          ⁡                      (            m            )                          =                  {                                                                                        ⁢                                      0                    ⁢                                          (                                                                        m                          =                          1                                                ,                        2                        ,                        3                                            )                                                                                                                                                              ⁢                                      1                    ⁢                                          (                                                                        m                          =                          4                                                ,                        5                                            )                                                                                                                              (        1        )            
Each selector of the selector circuit 3 selects a necessary signal among the signals R1 according to the source information included in the path information (x), and outputs it as the signal R2 (x, t)[x=1, 2, . . . , N].
In the case where the path information (1) to (N) are the values shown in FIG. 17, two paths are detected from each of the sectors 4 and 5. In this case, the selector 411 selects and outputs the signal R1 (4, t) as the signal R2 (1, t), the selector 412 selects and outputs the signal R1 (4, t) as the signal R2 (2, t), the selector 413 selects and outputs the signal R1 (5, t) as the signal R2 (3, t), and the selector 414 selects and outputs the signal R1 (5, t) as the signal R2 (4, t). Each of the selectors 415 to 41N selects none of the signals R1 (1,t) to R1(M,T). That is, the relationship between R1 (y, t) and R2(x, t) is represented by the following expression (2).R2(1,t)=R1(4,t) R2(2,t)=R1(4,t) R2(3,t)=R1(5,t) R2(4,t)=R1(5,t) R2(x,t)=0 x=5,6 . . . , N  (2)
The despreading circuit 4 despreads the signal R2 (x, t) with a despreading code independently for each path, and outputs a signal R3 (x, t) of a symbol rate (Fs). The code generated by the code generator 44 is used as the despreading code by adjusting the timing according to a delay value of the path information (x). If the code of code length L is C (k) and a timing function of each path based on the delay value is t (x), R3 is represented by the following expression (3). In the expression (3), t indicates t symbols.
                              R3          ⁡                      (                          x              ,              t                        )                          =                              ∑                          k              =              1                                      k              =              L                                ⁢                                          ⁢                      {                                          R2                ⁡                                  (                                      x                    ,                    k                                    )                                            ×                              C                ⁡                                  (                                      k                    -                                          t                      ⁡                                              (                        x                        )                                                                              )                                                      }                                              (        3        )            
For instance, in the case of the path information (1) in FIG. 17, the delay value included in the path information (1) is ‘2’. Therefore, the value of the function t(1) is ‘2’, and R3 (1, t) is represented by the following expression (4).
                              R3          ⁡                      (                          1              ,              t                        )                          =                                            ∑                              k                =                1                                            k                +                L                                      ⁢                                                  ⁢                          {                                                R2                  ⁡                                      (                                          1                      ,                      K                                        )                                                  ×                                  C                  ⁡                                      (                                          k                      -                      2                                        )                                                              }                                =                      ∑                          {                                                R1                  ⁡                                      (                                          4                      ,                      k                                        )                                                  ×                                  C                  ⁡                                      (                                          k                      -                      2                                        )                                                              }                                                          (        4        )            
The signal R3 (x, t) is outputted as the user received signal to a demodulation circuit (not shown) and also outputted to the ISSI estimation circuit 145 and the RSSI estimation circuit 6.
In the ISSI estimation circuit 145 of the conventional radio base station, the ISSI estimation is started with the preset value as the initial value. This initial value is zero or a value obtained by the station.
As the ISSI is dominated by noise components, it fluctuates little by little but does not fluctuate greatly in a long period of time. For this reason, the ISSI estimation is generally performed so that the change in the estimate is gentle and it becomes stable. Therefore, the estimate does not change from the initial value for a while from a start of the ISSI estimation. Thus, in the case where the initial value is zero, the estimate is a value close to zero for a while from the start of the estimation, and consequently the SIR value erroneously becomes a significantly large value. In that case, it is determined to be larger than the target SIR independently of actual link quality, and so the other station is requested to decrease the transmission power. And if the other station decrease the transmission power accordingly, the link quality is further deteriorated so as to cause a link break in the worst case.
In the case where the initial value is not adequately set, the ISSI estimation usually takes a long time, and so the SIR value during that time becomes incorrect. For instance, in the case where the SIR erroneously becomes high, there is a danger of the link break because of requesting the other station to decrease the transmission power, and in the case where the SIR becomes low inversely, there is a danger of increasing the interference to other users because of requesting it to increase the transmission power.
In the case where a value obtained in advance by the station is used as the initial value, it is necessary to change the value for each station so that management of the parameters becomes a huge amount of work. Moreover, it may not always be an optimum value since it is not a real-time value.
As described above, there is a problem that the transmission power control becomes unstable immediately after starting user communication in the case where the conventional radio base station estimates the ISSI used at the time of measuring the SIR of the received signal which is the reference of the transmission power control.